The present invention relates to Giant MagnetoResistive (GMR) devices in conjunction with micromachined beams to measure stresses with high sensitivity, and methods of making and using the same.
The GMR effect has been widely reported in multi-layer thin film sensors, where there are alternating ferromagnetic layers 12, made from materials such as Cobalt, Iron or Nickel, separated by nonmagnetic conductor layers 14, such as chromium or copper to form a sensor 10 such as illustrated in FIG. 1. When a current is passed along the length direction of the sensor, the electrical resistance of the multi-layer stack of films varies as the relative angle between the magnetizations of the individual ferromagnetic layers, as shown in FIG. 1. The resistance is minimum when the magnetization vectors between the neighboring ferromagnetic layers are parallel to each other, and is maximum when the two vectors are antiparallel to each other (at 180xc2x0), as shown in FIG. 2. It is to be noted that this is in contrast to the conventional AMR effect, where the resistance is maximum when the magnetization vector within a single magnetoresistive film is parallel to the direction of the current, and minimum when it makes an angle of 90xc2x0 to the direction of the current. The AMR effect is shown in FIGS. 3 and 4.
Typically, the change in electrical resistance of a GMR multi-layer stack for a full. rotation of the magnetization vector from a parallel to an antiparallel state can be anywhere from 2% to greater than 50%, and for the AMR effect, the change in resistance for a 90xc2x0 rotation is 1.5-3.5%.Therefore, if the magnetizations of some layers in a GMR multi-layer stack can be made to rotate under the application of a magnetic field, the GMR stack theoretically will provide a greater sensitivity magnetic field sensor than a conventional AMR film. However, one challenge in doing this is that the exchange coupling magnetic field between the alternating ferromagnetic layers in a GMR stack is very large, on the order of 2000 Oersted. As a result, to make an individual ferromagnetic layer rotate in relation to a neighboring ferromagnetic layer requires enormous magnetic fields. If the interlayer distance between the neighboring ferromagnetic layers is increased to reduce the exchange coupling field between the layers, then the GMR ratio (percentage resistance change) decreases correspondingly. As a result, it has not been possible to exploit the full extent of the classical GMR effect.
One approach to overcome the problem described above is the spin valve magnetic field sensor, a device that utilizes a version of the GMR effect. The spin valve essentially consists of two ferromagnetic layers 52 and 54 separated by a nonmagnetic conducting spacer layer 56 as shown in FIG. 5. Of these two layers 52 and 54, layer 54 is a xe2x80x9cpinned layerxe2x80x9d, in which the magnetization vector is pinned in one direction. The other layer 52 is a soft ferromagnetic layer, called the xe2x80x9cfree layerxe2x80x9d, whose magnetization vector is free to rotate in the plane of the film. The separation between the pinned layer 54 and the free layer 52 is chosen such that the coupling field between the two layers is not too large. In the quiescent state, the magnetization of the free layer 52 is oriented at 90xc2x0 to that of the pinned layer 54, as shown by the bold arrow in FIG. 5. Under the application of a relatively weak magnetic field, the rotation of the magnetic moment of the free layer 52 (as shown by the dashed arrows in FIG. 5) leads to a change in the relative angle of the magnetization between the pinned layer 54 and free layer 52, and results in a change in resistance of the device.
FIG. 6 depicts a typical resistance change of the device as a function of the applied magnetic field to the device. In the quiescent state, the resistance of the device is represented by the point X on the graph, and the change in resistance is linear for changes in magnetic field almost up to the point of saturation of the device as shown in FIG. 6.
The magnetization vector of the pinned layer in a spin valve device is usually held in place through antiferromagnetic exchange coupling between the pinned layer and a hard magnetic material (the xe2x80x9cpinning layerxe2x80x9d 58 shown in FIG. 5 for example) such as CrMnPd, PtMn, FeMn, NiMn, etc. Other methods to fix the magnetization of the pinned layer include permanent magnet biasing, current induced biasing, etc.
The classical Giant MagnetoResistive (GMR) effect, as it is described above, has also been contemplated as being used to measure mechanical strain induced by stress. This principle involves generating a rotation of the magnetization vector of the ferromagnetic film under the application of mechanical stress even in the absence of a magnetic field, which results in a resistance change of the film, which in turn can be used to infer the degree of stress. However, one still needs to overcome the large exchange coupling field between the alternating ferromagnetic layers, and in order to do this, it has been suggested to use an externally applied magnetic field to aid the rotation of one of the layers under an applied stress, and to measure the resulting change in resistance. However, in practice, this is very difficult to implement, since it is not possible to apply such large magnetic fields in sensors that are widely deployed in the field. Additionally, this method causes serious accuracy problems, since the effects of the externally applied magnetic field and the stress on the sensor need to be decoupled.
It has also been suggested, such as in U.S. Pat. No. 5,856,617, to use a in valve device of the type described above to measure strains in the cantilever tip of an atomic force microscope. In such a suggested strain gauge device, an example of which is illustrated in FIG. 5, the free layer 52 is made to be of non-zero magnetostriction, so that under zero magnetic field conditions, the free layer magnetization vector rotates under the application of stress to the cantilever beam, and the resulting change in relative magnetization vector angle between the free layer 52 and the pinned layer 54 leads to a resistance change in the device. The strain gage device is thus a conventional top spin valve, with the free layer 52 comprising an alloy of NiFe, Ni and Co and being deposited directly onto the substrate, a non-magnetic conducting layer 56 disposed between the free layer 52 and the pinned layer 54, and with the antiferromagnetic (AFM) layer 58 that is used for pinning the pinned layer 54 being on top of the stack. Although this device may find some use in measuring strains on atomic force microscope cantilever tips, there are several disadvantages to the use of this device as a general purpose strain gauge. The device""s drawbacks relate mostly to the performance, reliability and processing limitations that are inherent with this type of design, and are listed below.
First, since an antiferromagnetic (AFM) layer 58 is used to pin the pinned layer 54 through exchange coupling, the device is subject to reliability concerns, since extended exposure of the AFM material to elevated temperatures around 150-200 C can cause xe2x80x9cdepinningxe2x80x9d of the pinned layer, which destroys the effectiveness of the sensor. This is especially true if the antiferromagnetic material that is chosen has a low xe2x80x9cBlocking temperaturexe2x80x9d (the temperature at which the antiferroniagnet starts to lose its exchange anisotropy). Furthermore, if the AFM material chosen is one that needs high temperature annealing, this introduces other processing problems such as the compatibility of the film with the substrate on which the multi-layer stack is being deposited, due to thermal mismatch concerns and delamination of the stack. Moreover, most manganese based AFMs have poor corrosion resistance.
Second, it is very difficult to maintain the magnetization of the free layer 52 to be pointing in a direction that is at 90xc2x0 to the pinned layer 54 in the quiescent state. There are several competing magnetic torques that affect the net quiescent state magnetic moment of the free layer, including the intrinsic stresses, the shape demagnetizing fields, current induced fields, interlayer coupling fields with the pinned layer 54, and finally the intrinsic anisotropy of the free layer 52. It is very difficult to balance all these moments to arrive at a final moment vector that is pointed at 90xc2x0 to the pinned layer 54. This is especially true at the edges of the sensor. As a result, there is a corresponding reduction in the sensitivity, and error in the linearity and offset of the device.
Third, using the configuration such as described above, there are several problems associated with stability of the device, since the free layer 52 is magnetically very soft. During the measurement of stress and strain, even very small magnetic fields in the vicinity of the device can affect the rotation of the free layer magnetization. This is further compounded by problems at the edges of the sensor, where stress effects and other demagnetization effects apply torques on the magnetization vector of the free layer 52. Therefore, it is desirable to have a free layer 52 that is relatively insensitive to small extraneous fields.
Fourth, it is difficult to process the device and deposit the multi-layer thin films on specialty substrates, such as Teflon or other flexible substrates like kapton on which the strain is to be measured. This is due to the fact that most AFMs require elevated temperature annealing steps to get the required magnetic properties. Elevated temperature annealing on such specialty substrates is likely to introduce severe stresses due to thermal mismatch, and peeling due to lack of adhesion.
Fifth, it is very difficult with this sensor to separate out the intrinsic effects of the strains developed within the films due to thermal mismatches between the substrate and the device during elevated temperature processing or operation of the device. Since the free layer is extremely sensitive to stresses, and it cannot separate between an externally applied stress and an intrinsic stress introduced during processing, linearity may be lost due to processing stress, and random offsets maybe encountered in the field due to varying environmental conditions.
Sixth, this strain gauge is limited to realizing a total GMR response of about 3-5%. Noting that this response corresponds to a complete 180xc2x0 relative change in angle between pinned and free layer magnetization vectors, the design described allows only half of the total GMR effect to be realized, since the maximum difference in angle between the pinned and free layer magnetizations between the quiescent state and the fully stressed state at zero magnetic field is only 90xc2x0, and not 180xc2x0. Therefore, it is unlikely that one can obtain a resistance change greater than 3-4% with this design.
Seventh, if the device is used to measure both tensile and compressive stresses simultaneously, the maximum signal output is reduced to one fourth of the total GMR response for either sign of stress, and linearity of the response is compromised.
Eighth, it is difficult to separate the temperature induced drift in the sensor due to an inherent material characteristic called the xe2x80x9ctemperature coefficient of resistance (TCR)xe2x80x9d. The TCR denotes the change of quiescent resistance of the device at zero field, zero stress conditions as a function of the temperature. Typically, this number is about 0.15-0.2%/xc2x0 C. for GMR sensors. When the sensor is being used as a strain gauge, however, it is difficult to separate out the resistance change of the sensor into ambient temperature induced effects and stress induced effects. As a result, strain gage measurement circuitry usually involves elaborate circuitry to compensate for the temperature induced changes in the sensor. A sensor such as the one described is subject to the same drawbacks as other typical piezoresistive sensors available today.
For all the reasons described above, it is necessary to make substantial improvements in order to realize a stress sensor that has high sensitivity (large response), good stability, and good reliability.
The fingerprint sensing industry uses several different technologies to image fingerprints. The two most prominent technologies are optical based sensors and capacitance based sensors. Optical sensors use a light source, lenses and a prism to image the xe2x80x9cridgesxe2x80x9d and valleys on a fingerprint, based on differences in the reflected light from the features. The conventional capacitance sensor uses semiconductor type processing to fabricate a two-dimensional array of capacitors. The individual sensors form one plate of the parallel plate capacitor, while the finger itself, when placed on the array, acts as the second plate. Upon contact with the array of sensors, the individual distance from each sensor to the skin is measured using capacitive techniques. The difference in distance to skin at the ridges and valleys of a fingerprint provide the means to replicate the fingerprint. An example of the use of capacitive sensors to measure the spacing is shown in FIGS. 7A and 7B.
Both the above techniques fundamentally measure the spacing between the fingerprint features, and the sensor. The measurement of spacing is inherently subject to several distortion effects: since the height difference between the ridges and valleys is only of the order of 50 microns, any parameter which affects the spacing between the finger and the, sensor will affect the measurement. For example, both types of sensors are very sensitive to the thickness of the protective coating. They are also sensitive to oils or grease on the finger, and the presence or absence of moisture on the finger. In addition, most of these sensors are affected by the ambient temperature at the time of sensing. Under very hot or very cold conditions, the capacitive sensor can provide erroneous readings. The combined effect of all these variables results in a very distorted image of the fingerprint, as shown in FIG. 7C.
As a result of the above drawbacks to spacing based reproduction of fingerprints, it would be very useful to be able to use the difference in pressure exerted by the ridges and valleys of a fingerprint on a sensor to replicate the fingerprint image. In principle, a pressure based fingerprint sensor would be impervious to the drawbacks listed above, such as wet or dry conditions on the fingertip, presence of oil or grease on the fingertip, thickness of protective coatings, etc, and would produce a xe2x80x9cdigitalxe2x80x9d response, depending on whether the sensor experiences a ridge or not. This situation is illustrated in FIGS. 7D and 7E, where the pressure sensor can highlight only the ridges, which are the lines of interest in a fingerprint. However, due to a variety of factors, including the very low sensitivity and inability to provide the required resolution, pressure based sensors have not been deployed for the replication of fingerprints.
Accordingly, there remains a need for a device suitable for use as a stress and/or pressure sensor that has high sensitivity yet can provide high lateral resolution. Moreover, there further remains a need for a sensor that is suitable for use in fingerprint identification and verification that is less sensitive to adverse conditions such as extreme temperatures and skin oils and grease.
It is an object of the present invention to provide an improved GMR sensor.
It is another object of the present invention to provide a GMR sensor that does not require an additional layer to pin any ferromagnetic layer, but instead uses an external current source to properly bias the ferromagnetic layers.
It is another object of the present invention to provide a GMR sensor that allows both ferromagnetic layers to freely rotate, thus increasing the dynamic sensing range of the GMR sensor and allowing the entire GMR response to be sensed.
It is a further object of the present invention to provide a GMR sensor capable of sensing both compressive stress and tension.
It is a further object of the present invention to provide a GMR sensor that can be adapted to have substantial independence from temperature shifts.
Another object of the invention is to provide a GMR sensor that is suitable for use in fingerprint identification and verification.
Another object of the invention is to provide a GMR sensor that is suitable for use in fingerprint identification and verification and that is less sensitive to adverse conditions such as extreme temperatures and skin oils and grease. Another object of the invention is to provide a GMR sensor that is suitable for use in fingerprint identification and verification and that is less sensitive to transient ESD voltages, and also mechanical abrasion.
The present invention fulfills these and other objects of the present invention by providing a pressure sensing device that includes at least one GMR sensor, and preferably an array of GMR sensors, with each GMR sensor having a conducting spacer layer interposed between two ferromagnetic layers. In an unbiased state, the magnetization vector of each of the ferromagnetic layers is preferably parallel to each other. Upon application of a current, however, the magnetization vector of each ferromagnetic layer is changed, preferably to an antiparallel position, in which state the sensor is used to then sense stress applied thereto. Upon application of stress, the magnetization vectors of both free magnetic layers will rotate, thus causing a corresponding and proportionally related change in the resistance of the magnetic material of the sensor. This change in resistance can be sensed and used to calculate the stress applied thereto.
While the above provides an overview of the invention, there exist numerous other significant aspects and advantages that will become apparent in the discussion provided hereinafter.